The invention relates generally to condensing furnaces, and more particularly, to a condensing heat exchanger of such condensing furnaces.
Conventional condensing furnaces include a primary heat exchanger and a condensing heat exchanger arranged adjacent one another and generally aligned with an air source. Air from the air source blows first across the outside of the condensing heat exchanger and then over the primary heat exchanger, such that heat transfer between the fluid in the heat exchangers and the air occurs through convection.
Forcing airflow through a condensing heat exchanger having a plate fin and tube construction requires a given amount of power consumption (watts) by the fan or other air source. To minimize the power drawn by the air source, the condensing heat exchanger may be widened to increase the area over which air from the air source flows. The size of the condensing heat exchanger is limited, however, because after passing over the condensing heat exchanger, the airflow subsequently flows over the adjacent primary heat exchanger, which has a more narrow flow profile. By increasing the width of the condensing heat exchanger relative to the primary heat exchanger, a wider distribution of the airflow will be achieved. This wider distribution of airflow will allow more air to pass around the outside of the primary heat exchanger such that the interior of the primary heat exchanger will have a higher temperature. Baffles can be added to the side of the primary heat exchanger to limit the amount of bypass airflow, but in exchange they will also increase power consumption from the air source. This increase in power consumption can negate the reduction in power consumption achieved by the widening of the condensing heat exchanger.
Construction of a plate fin and tube heat exchanger generally includes equally spaced tubes that lace through at least one plate fin. The holes in the plate fin have extruded collars to provide proper fin spacing and an increased area for heat transfer from the tube to the collar. The collars on the fins may encompass the entire length of the tubes by nesting with one another, and create a barrier to the airflow such that air has to divert around the tubes and collars to flow through the condensing heat exchanger coil. As a result of this increased resistance to the flow, the power consumption of air source is similarly increased.